1. Field of the Invention
The present invention concerns a method for focus adjustment in a CT (computed tomography) apparatus of the type having at least one x-ray tube with a springing focus, that x-ray tube rotating around a system axis, and a multi-line detector with a plurality of detector elements, is situated opposite the x-ray tube.
2. Description of the Prior Art
CT apparatus of the above type is known wherein the focus on the anode changes position in one dimension or in two dimensions with a specific focus jump frequency. The output signals of the detector elements of the detector are integrated with a specific detector sampling frequency. The focus jump frequency is equal to the detector sampling frequency; and the jumped-to focus positions and the phase shift are set between the focus jump frequency and the detector sampling frequency.
It is generally known that a springing focus arrangement can be used to improve the spatial resolution of a CT apparatus. Using a magnetic deflection system in the x-ray tube, the radiation-generating electron beam is influenced such that a plurality of positions on the anode can be jumped to with a specific springing focus frequency, and thereby each detector element can be irradiated from different directions and exposed multiple times in a projection. The sampling of the detector signals ensues with a frequency that results from the plurality of the projections per rotation of the gantry, the rotation speed and the plurality of the different springing focus positions. If the sampling ensues with suitably shifted focal point, the effective sampling rate can be multiplied and the resolution and quality of the CT examination can be improved.
The focus deflection can be implemented both in the axial direction (Z-direction=system axis direction) and in the transaxial direction (φ-direction=circumferential direction=azimuthal direction) or in combinations of both directions. In each case it is necessary to determine the phase relation between the deflection movement of the focus (focus springing frequency) and the frequency of the data acquisition (detector sampling frequency), the deflection difference in the springing focus positions and the absolute focus position on the anode (focus offset).
For example, by correct selection of the deflection of the springing focus (focus offset) in the azimuthal direction two positions are achieved (known as focus alignment in the rotation center of the gantry) as well as a quadruple sampling of the detector pixels in the rotation center in connection with an azimuthal springing focus and a known ¼ offset of the detector elements. A transfer frequency of the imaging system (focus/detector system) that is four times higher is thus enabled.
For this purpose it is necessary to carefully match the alignment of the springing focus positions and the phase shift between the focus springing frequency and the detector sampling frequency. In principle, these quantities can be determined from CT scans with orbital sampling and a suitable phantom in a springing focus operation. An eccentrically positioned, strongly attenuating small sphere or (if the springing focus jumps only in the azimuthal direction) a small cylinder is suitable as a phantom. This matching is conventionally achieved by iterative approximation of the optimal deflection of the focus and the optimal phase with multiple measurements and intervening evaluations.
Such iterative methods are very time-consuming and costly and there is need to simplify and shorten such procedures.